In a moment of weakness (and positive bank account), I bought a used Canon 800mm f5.6. The first two images i took hand held of all things when i first got it and used my 1DX. The leopard and black shaheen were taken recently this year. I was resting on my knee for the leopard using the 5DSR at 3200 ISO at 1/80th sec shutter as the light was so poor in the forest canopy and the black shaheen falcon I was dangling over a cliff edge resting the lens on my arm on the rocks and was also using a 1.4X TC for added reach so I had 1120mm focal length at 1/2000th shutter and ISO 640.
I'm pleased with the results the 800 can achieve and it reaffirmed I made a good choice even though I own a 600mm which seems a duplication at first. This said I'm using the 800mm more than i thought i would ...and not just on tiny birds.